The present invention relates to apparatus and methods for burning wood to produce heat, e.g. wood-burning stoves, and more particularly to arrangements and methods employed therein for controlling the rate of burning of the wood.
With the increases of recent years in the cost and periodic shortages or unavailability of traditional heating fuels such as heating oil and natural gas, many less conventional methods of space heating have correspondingly grown in popularity. Wood, as a relatively inexpensive, plentiful and efficient alternative heating fuel, has resultingly enjoyed a veritable rebirth in use as a heating fuel, accompanied by a proliferation of manufacturing concerns producing and selling wood burning stoves. However, in addition to its advantages, the use of wood as a fuel also presents certain problems not ordinarily encountered with more traditional fuels. In the ordinary, uncontrolled burning of a given quantity of wood, three basic stages of burning progressively occur: an initial stage during which water in the wood is evaporated, a subsequent stage during which volatile materials in the wood are combusted, and a final stage during which charcoal resulting from the second stage is burned. Over the course of the three stages, the temperature and, accordingly, the heat output of the combustion gradually rise to a maximum temperature which is maintained for a relatively short time after which the temperature and heat output quickly decrease. As will therefore be understood, a general problem exists in the burning of wood for space heating purposes of modulating the temperature and heat output of the burning wood over the total time period of the combustion process.
As is known, wood burning stoves, in employing an enclosure for combustion therein of wood into which the flow of combustion air may be regulated, provide some control over the combustion process. Conventionally, several approaches have been employed in the design of wood burning stoves in attempting to solve or mitigate the above-described problems. According to one such approach, a wood burning stove is provided with a manually adjustable damper to permit the regulation of the quantity of combustion air admitted to the stove thereby to control the rate of burning. However, although stoves having dampers of this sort are effective in lengthening the period of time over which a given quantity of wood will burn as contrasted to the uncontrolled burning thereof, the burning nevertheless follows the three above-described stages and, accordingly, a common complaint of such stoves is the overheating which results during the second stage of burning.
In a conventional modification of this manually controlled damper arrangement, the damper is operatively connected to a thermostat mounted exteriorly on the stove for controlling opening and closing movements of the damper in response to changes in the temperature of air surrounding the stove and radiantly heated thereby. While stoves of this sort have been found to be relatively effective in generally modulating the heat output of the stove over the period of burning of a given quantity of wood and are advantageously inexpensive, such control arrangements, as a result of their disposition exteriorly of the stove in relatively still air and their reliance upon changes in the temperature of such still air thereat, experience a relatively substantial time delay in responding to changes in the internal temperature of the stove which, accordingly, produces a cyclic variance of relatively broad range in the heat output of the stove. Problems are experienced in such stoves in maintaining combustion of the wood at damper settings providing for minimal combustion air intake for low-level burning without extinguishing the fire from lack of air since the response time of the thermostatic damper control is generally not sufficiently quick to actuate opening movement of the damper once the thermostat senses the need for additional combustion air to modulate a decrease in the combustion temperature and, therefore, such stoves must ordinarily be operated utilizing a greater than desirable mean rate of air intake to offset this effect of the delayed response of such stoves, disadvantageously causing more rapid burning of the wood and overheating.
In contrast to the above, the present invention provides an improved thermostatic damper control arrangement and method for wood burning stoves and the like particularly adapted for actuating opening and closing movements of the air intake damper arrangement of such a stove quickly and accurately in response to the temperature and rate of burning of wood in the stove thereby permitting both the modulation of the heat output of the stove with reduced cyclic variation and little or no overheating, and the operation of the stove utilizing unconventionally low rates of air intake.